The present invention relates to a suction controlled gear ring pump comprising a housing, an internally geared hollow gear rotatably arranged in a gear box of the housing, a pinion having one tooth less than said hollow gear engaging with and arranged in said hollow gear, the teeth of said pinion forming, together with the teeth of said hollow gear alternately expanding and reducing successive feed cells for the operating liquid and providing sealing between said feed cells, inlet and outlet ports arranged in the housing for the entry and discharge of the operating liquid, said ports opening out into the gear box on either side of the location of deepest tooth engagement, a fixed or variable throttle provided in the inlet port, and check valves in the pressure region of the pump. As a rule, the drive of the pump is effected by the primary shaft bearing the pinion. For example, such pumps are used for feeding hydraulic systems. The invention especially relates to the use of such a pump as an oil and/or hydraulic pump for automobile engines and transmissions.
Especially automobile engines and transmissions are operated at high rotating speeds. The nominal values of the rotating speed can be 10 : 1 and above.
In contrast, the targeted output of the lubricating feed mechanism of an automobile engine which, in the case of automatic transmissions, additionally has to assume the function of the pressure supply to the hydraulic shift elements and the feeding of the converter against cavitation, is approximately proportional to the rotating speed only in the lower third of the operating range both as far as engines and transmissions are concerned. In the higher speed range, the oil requirements increase far more slowly than the rotating speed of the engine. What would be necessary, therefore, is a drive controlled lubricating or hydraulic pump or a pump providing a feed volume which can be adjusted according to the rotating speed. The most common form of such an oil and/or lubricating pump is the gear ring pump, because it is simple, inexpensive and reliable.
The disadvantage is that the feed output (per rotation) is not adjustable, i.e. the theoretical feed quantity is proportional to the rotating speed. The practical characteristics of the feed quantity versus the rotating speed depend on a number of parameters such as feed pressure, oil viscosity, flow resistance in the suction and pressure conduit, teething configuration of the gears, width of the gears and construction of the pump. In most cases, adjustment of the feed line to the consumption line, for example of an internal combustion engine, is too costly, and for this reason a bypass valve is used which by feedback control reduces the excess oil supplied in the case of excess feed at a certain set feed pressure and channels it back into the suction line in decompressed form. Consequently, this type of control results in considerable losses in the control line so that the effectivity decreases in a undesirable way as the rotating speed increases. The only practicable way to avoid this excess quantity which occurs above a certain rotating speed of the pump is suction control. Since the flow resistances increase overproportionally as the oil speed increases, the static pressure in the suction opening of the gear box decreases more and more until the so-called cavitation pressure threshold has been reached, i.e. until it falls below the vapour pressure of the oil. The cell content then consists partly of liquid oil, partly of oil vapour, and partly also of sucked-in air, said cell content being under a static pressure which is significantly below the atmospheric pressure. It is no problem to determine or to control the flow resistances in the suction pipe, for example by using correspondingly narrow suction pipes or a shutter or by regulation with a suction gate valve, in such a way that a high degree of adjustment of the feed quantity of the gear ring pump to the requirement line of the consumer is achieved.
The occurrence of cavitation is the disadvantage of this type of control. For if the cell content which is under a low absolute pressure and consists partly of liquid and partly of gas is abruptly transferred into zones of higher pressure, as is system-inherent for such pumps, the gaseous components of the cell content implode with such force that undesirable sounds or, even worse, destruction of the cell walls result.
If a volumetric pump of this type is to be adjusted by throttling on the suction side, then such implosions must be avoided. To achieve this, one uses the known method, namely one provides the cell content on the positive displacement side of the pump, i.e. in the range of the diminishing cells, with sufficient time to sufficiently increase the static pressure by gradual compression in such a way that no implosions of gas bubbles can occur any more at the moment the cell is connected with the discharge port, because said gas bubbles have once again condensated into liquid due to the continuous decrease of the cell volume or have dissolved in the liquid (for example air). Constructively, this solution can be obtained in the most compact way with an internal geared wheel pump where the individual feed cells are separated from each other sealingly. From a construction point of view, the timespan for the slow compression of the vapour and air spaces is assured by the fact that the cells on the displacement side of the pump are at first only connected with the feed pressure space by check valves so that the feed pressure cannot become effective if the cell is not completely filled with liquid.
However, if the cells are already filled completely with liquid on the suction side which, as illustrated above, is the case at lower rotating speeds, then the higher squeeze pressure in the cell opens the check valve towards the pressure feed space so that, at an only slightly increased cell pressure as compared to the feed space according to the opening pressure of the check valve and the flow resistance of said check valves. One such construction is known from German Patent Specification 30 05 657 (U.S. Ref. 4,443,169). In that construction, axial bore holes extend over the entire pressure half of the pump in the housing towards the discharge port, said axial bore holes containing, at a distance from the gear chamber, check valves which open only, if the pressure of the cell respectively positioned in front of the relevant bore hole exceeds the pressure in the discharge port. Accordingly, this pump has a large axial extension. The spring valves used there can break. Another disadvantage is the uneven connection of the feed cells to the discharge port. Finally, the pressure distribution is disadvantageous with regard to the occurrence of the cavitation-induced implosions.